Add TensorFlow Sandbox resource to README

Adds the TensorFlow Sandbox repository as a Python entry in Build Your Own X.

This repo demonstrates stepwise ML projects:
- Module 1: Basics - tensors, variables, reshape, preprocessing
- Module 2: Classification - Titanic & Iris datasets
- Module 3: CNNs - CIFAR-10, data augmentation, transfer learning
- Module 4: Reinforcement Learning - FrozenLake Q-learning

Includes saved `.keras` models for evaluation or further experimentation. Focused on learning by building and understanding TensorFlow end-to-end.  

This entry showcases an end-to-end educational ML sandbox suitable for BYOX readers.

README.md

@@ -249,6 +249,7 @@ It's a great way to learn.
 * [**JavaScript / Java**: _Neural Networks - The Nature of Code_](https://www.youtube.com/playlist?list=PLRqwX-V7Uu6aCibgK1PTWWu9by6XFdCfh) [video]
 * [**JavaScript**: _Neural networks from scratch for JavaScript linguists (Part1 — The Perceptron)_](https://hackernoon.com/neural-networks-from-scratch-for-javascript-linguists-part1-the-perceptron-632a4d1fbad2)
 * [**Python**: _A Neural Network in 11 lines of Python_](https://iamtrask.github.io/2015/07/12/basic-python-network/)
+* [**Python**:_Build Your Own TensorFlow Models_](https://github.com/aypy01/tensorflow) - A modular guide to mastering TensorFlow: from basic Tensors and classification to CNNs and Reinforcement Learning.
 * [**Python**: _Implement a Neural Network from Scratch_](https://victorzhou.com/blog/intro-to-neural-networks/)
 * [**Python**: _Optical Character Recognition (OCR)_](http://aosabook.org/en/500L/optical-character-recognition-ocr.html)
 * [**Python**: _Traffic signs classification with a convolutional network_](https://navoshta.com/traffic-signs-classification/)